Probably the best known example of the use of the behavior of a penetrating body in a material to determine its properties is the measurement of material hardness. Here there exist a considerable number of methods, for example, Shore hardness measurement, Vickers hardness measurement, Rockwell hardness measurement, Brinell hardness measurement, Herberts pendulum etc.. In view of the fact that the surface layer is fully or partially destroyed, however, these methods cannot be used in a great many areas of application. For example, if the hardness of paint is to be measured, it may be the case that the paint layer is not completely penetrated. Despite this, the paint is damaged and it is possible that a rust spot at the test point could form the nucleus of corrosion, whereas the rest of the paint layer is still in order. For this reason, the finished object is not measured, rather a test specimen. The test specimen, however, is not the finished object.
There are also non-destructive methods for measuring the properties of solid materials which can be derived from their behavior under penetration. For example, it is known that, by placing an ultrasonic probe on a material, the frequency mismatch can be measured. This method is very sensitive to the degree of coupling with the test object. In addition, it is not the hardness alone which is measured, because the measured value depends substantially on the modulus of elasticity of the material, the thickness of the coating and the shape of the object, in particular, on the thickness. The method can only be used for comparative assessments and not for absolute measurements.
A further non-destructive method giving absolute measurements was described in the "Industrie-Anzeiger" of 2nd Dec., 1981 under the (translated) title "Method for testing the case hardness distribution of forgings". The device described is very expensive. It is also so large that the object must be brought to the device; the reverse is not possible. A force is applied inductively and the depth of penetration is measured by optical displacement measurement. Due to the sensor system employed for measurement, it is not possible to safely determine the depth of penetration in the micrometer range. The method is designed for measurement of case hardness with typical penetration depths of 500 um. Similar to the case of a micrometer screw gauge, the system requires a very rigid yoke. The depth of penetration at a given force is converted by a difficult-to-use nomogram, since the measured value is a complicated function of the force applied.
An apparatus has been described in the German Patent Application P No. 34 08 554.8 corresponding to U.S. patent application Ser. No. 606 922, the Japanese Patent Application No. 75655/84 and the English Patent Application No. 840 9155 which is inexpensive to manufacture, which is so small that it can be brought to the place of use, which measures non-destructively, which allows reproduceable, absolute measurements, which can also actually measure the hardness of the topmost coating in the case of inhomogeneous materials, which leads to simple functional relationships, which has no exotic system such as ultrasonic sensors, interference measurements or similar whatsoever and can also be used by unskilled personnel.
The invention specified there has the following advantages:
(A) The probes, which are already known, allow very precise measurement of coating thickness over a wide range of thicknesses. In view of the fact that very precise absolute measurements are possible, it is also possible to measure coating thickness differences to high precision. For example, for a coating thickness of 20.mu.m, a change in coating thickness of 0.05.mu.m is easily detected. Alternatively, in the range from 100 to 500.mu.m a change of coating thickness of 0.1.mu.m is measured with ease. Such small changes in coating thickness can be generated by extremely low contact forces. To give an idea of the size of the necessary forces, these lie, for example, in the range of 0.05 to 1N.
(B) These minimal forces do not destroy even the softest coating.
(C) In view of the fact that the necessary forces are very low, there is no danger that the device will bend under its own weight and it is possible to make it rigid enough for the purpose here described with very little expenditure of material.
(D) With such low forces, measurements are actually only made in the surface zone which, in any case, provides the most important testimony in most applications.
(E) With such small measurement areas the measurements themselves leave no traces whatsoever.
(F) In view of the fact that high energies need not be applied high energies need not be supplied either. It is, therefore, perfectly plausible to use the device with battery power.
(G) The measurement is practically displacement-free, with all the resulting consequences.
(H) The device can be used to measure on very small and/or curved surfaces.
Probes which measure such thick coatings with high precision have been known for many years, and are described, for instance, in German Utility Patent No. 72 43 915, German Utility Patent No. 73 36 864, German Patent Disclosure No. 25 56 340 or the still to be published German Patent Application P No. 33 31 407.
The invention described in German Patent Application P No. 34 08 554.8 has, however, the following disadvantages:
(A) The coating to be measured must be deposited on a substrate, such as, for example, in FIG. 2 the coating 22 is deposited on the substrate 23. If probes which are based on the magnetic method are used, then, the coating must be deposited on soft iron. If probes which are based on the eddy-current method are used, the coating must be deposited on, for example, aluminium or a similar metal. It is, however, desirable that materials which are not deposited on a substrate can be measured. It is true that, for instance, paints are very frequently applied to magnetic or non-magnetic substrates. In many other cases, however, this is not so.
(B) The probes must be specially manufactured because their tips are at the same time the penetrating body.
(C) Depending on whether the coating is deposited on a magnetically soft material or a non-magnetic material either one type of probe or another type of probe must be used. The expert who is, for instance, only interested in measuring the hardness or the flow behaviour of such materials often does not know what substrate lies underneath the coating to be measured and very frequently has absolutely no or only a very hazy knowledge of the magnetic properties of the substrate.